Rivet squeezer

ABSTRACT

A tool to preload and upset rivets being installed in structure including a preload piston with a connected preload sleeve to establish a predetermined preload between the structure and the rivet, an anvil, and a squeeze piston with a connected head set for upsetting the rivet against the anvil once the preload has been established.

United States Patent 11 1 1111 3,747,194 Christensen July 24, 1973 RIVETSQUEEZER 2,797,596 7 1957 Seely 29 24354 3,557,442 1 1971 Speller 2924353 [751 lnvemor- Chmlensen, Long Beach, 3,581,373 6/l97l Murdoch 29243.54

[73] Assignee: McDonnell Douglas Corporation, Primary Examiner-Robert C.Riordon Santa M nica, Ca if- Assistant Examiner-J. C. Peters [22] Filed.Oct 4 1971 Attorney-Walter Jason, George W. Finch et ul.

. I [2]] Appl. No.: 186,048

[57] ABSTRACT 52 U.S. c1 29/243.s4 72/391 72/465 A to Preload and "Privets being installed in 511 1m. 01 nzi /22 Structure including PPiston with a connected [58] Field of Search 29/243.52 243.53, Preloadsleeve to establish a predetermined Preload 29 241354. 72 3 391 39 21604 5 tween the structure and the rivet, an anvil, and a squeeze pistonwith a connected head set for upsetting 56] References Cited the rivetagainst the anvil once the preload has been es- UNITED STATES PATENTStabl'shed 2,539,419 1/1951 Harcourt 29/243.54 7 Claims, 5 DrawingFigures RIVET SQUEEZER BACKGROUND OF THE INVENTION As larger high stressstructures have been built, new fasteners have been required to improvethe fatigue life of such structures. One such fastener, covered in partby U. S. Pat. No. 3,426,641, is the Rivbolt developed by the MarquardtCorporation and available'from Voi- Shan Company, Culver City, Calif.Rivbolts are capable of producing quality riveted joints of the highestshear strengths while producing significant improvements in the fatiguelife of the structure into which they are inserted. These Rivbolts canbe formed of high strength materials which are ordinarily difficult tosqueeze form without cracking, yet the Rivbolts can be squeeze drivencrack-free with precision quality because of a controlled upset of thedriven Rivbolt and the utilization of a special collar and rivet setadaptor tooling. Examples of some of the materials that can be now usedin Rivbolts include solution treated and fully aged alloys such as 6-4and Beta III titanium, A-286 Waspaloy and 2024 aluminum. The controlledupset of the Rivbolt prevents structural deformation in the area of thedriven head thereof and produces uniform and substantially equal Rivboltswelling in the hole through which the Rivbolt is driven. The resultofusing Rivbolts is an optimum strength, riveted joint with high fatiguecapability and minimum bowing or built in stress along the length of theriveted structure. These latter advantages enable substantialweightreduction and increased payload along a joint.

The Rivbolts have heretofore been installed with large, stationary orcrane suspended riveting tools. Such tools usually include a singlehydraulic assembly which first establishes a controlled preload from thehead of the Rivbolt across the structure to a collar placed about thetail of the Rivbolt. Establishing the proper preload before the Rivboltis upset is critical since too little or too much preload results in aninferior joint. Once the preloadin the structure has been established,the hydraulic assembly is used to apply force to the tail of theRivbolt. The force causes the body of the Rivbolt to swell within thehole to expand the diameter of the hole to a predetermined degree,thereby stress coining" the walls of the hole. As the force is increasedto a predetermined terminal value, the tail of the Rivbolt is mushroomedout over its collar, securely lockingthe Rivbolt in place. The prior arttools usually employ large mechanical springs or complex Hydra-Springdevices in conjunction ,with their hydraulic assemblies to establish thepreload. Such spring.

devices have manydisadvantages in p reloading applications because theymust be relativelylarge tosatisfy high force requirements, they canprovide only a limited stroke, and the preload force. they establish isundesirably variable through the stroke thereof. Furthermore, a numberof different spring devices must be stocked in order to provide thevarious preload forces and strokes which are required for installingRivbolts of different lengths or different diameters. Foreach Rivboltchange, the prior art tools must usuallybe disas-v sembled to installthe appropriate preload spring device.

SUMMARY OF THE INVENTION The present rivet squeezer toolisprimarilydesigned for installing rivets of the Rivbolttype. The toolis usually hydraulic powered and it may be a hand tool shaped in thegeneral form of a C-clamp with separate preload and squeeze cylinders atthe opposite ends. After the Rivbolt has been inserted through a hole inthe structure stack-up to be joined and the collar has been positionedabout the Rivbolt tail, a preload sleeve is slipped over the tail of theRivbolt and is pressed firmly against the collar to surround it andrestrain it within the tool. The preload sleeve is normally biased in anextended position by a connected preload piston in the pressurizedpreload cylinder. The preload pressure is set at a predetermined valueto match the particular Rivbolts specified preload requirement.

Controls for the squeeze cylinder are then energized so a squeeze pistontherein advances to extend a connected head set into contact with thehead of the Rivbolt. As the squeeze piston continues to advance, thehead set gradually builds up force through the structure stack-up untilthe specified preload as controlled by the pressure in the preloadcylinder acting on the preload piston has been achieved. At this pointin the riveting cycle, the preload force on the preload piston isovercome and the preload sleeve and the preload piston are driven backagainst the constant preload pressure. When the tail of the Rivboltcomes in contact with a post anvil which extends through the preloadsleeve, a very rapid force build up occurs until the elastic limit ofthe Rivbolt is exceeded. This causes the Rivbolt to flow and swellwithin the hole in the structure, actually expanding the walls of theholeto increase the fatigue strength of the structure. With continuingbuild up of force, the tail'gradually mushrooms, increasing in diameteruntil it achieves a shape commensurate with the terminal upset forcedesigned into the Rivbolt. The present tool is usually constructed sothat it can be regulated to apply no more than the desired terminalupset force.

After upset has been accomplished, the tool is deenergized and thepressure in the squeeze cylinder is relieved so the head set moves backto its retracted position which enables the preload sleeve to return toits initial extended position. The tool is then removed from about thecompleted Rivbolt connection. The assistance of optional stripping meansmay be employed to force the preload sleeve off of the mushroom deformedtail of the Rivbolt although this is usually not necessary.

The riveting portion of the present tool is designed to be relativelylightweight for handling ease and portability and also to be relativelysmall in size so it can be used in tight quarters. The two oppositeoperating cylinders are used to handle the separate functionalrequirements of a Rivbolt tool without resort to the unneededcomplication of employing co-axial pistons. The use of two separatecylinders also enables the con struction-of'a tool having a relativelylong stroke to facili tate engagement into difficult structuralconfigurationsand'to accommodate wide variations in structure stackthicknesses and .in tail lengths of the Rivbolts.

The .means to supply the preload pressure to the preload cylinder areusually a device such as a gas-fluid accumulator. Such accumulatorsinclude a gas chamber and a.fluid chamber which are separated from eachotherby, a.flexible diaphragm. Chosen settings of the gas pressurecharge in the accumulator correspondingly set the fluid pressure thereinwhich is conducted to the rivetingportion of the tool to establish thecontrollable preload force. The force is made relatively constant byproviding an accumulator whose gas volume is much larger than the volumeof the preload cylinder. The squeeze cylinder, on the other hand, can beprovided with relatively high operating pressures by means of such as acontrollable air-oil pump. Both the accumulator and the pump can berelatively small, portable devices. This is desirable so they canaccompany a tiveter using the tool into tight places, such as inside anaircraft wing, so that only short high pressure hoses are required tosupply the desired pressures from the pump and accumulator to theriveting portion of the tool.

It is therefore an object of the present invention to provide a.tool forinstalling Rivbolt type fasteners.

Another object is to provide a rivet squeezer with adjustable preloadforce controlling means that do not vary through the stroke range of thesqueezer.

Another object is to provide a rivet squeezer with a relatively longstroke capacity.

Another object is to provide a portable rivetsqueezer of small size andweight.

Another object is to provide a rivet squeezer which is relativelyeconomical to build and maintain.

Another object is to provide a rivet squeezer which is safe andfoolproof.

Another object is to provide a rivet squeezer whose source of externalpower may be relatively low pressure air, low pressure hydraulic fluidand/or electricity.

Another object is to provide a rivet squeezer whose preload does notdepend on a mechanical or an air-oil internal spring.

Another object is to provide a Rivbolt squeezer which can be made inpractically any size and capacity so that it is adaptable over a widerange of size and length Rivbolt configurations.

These and other objects and advantages of the present invention willbecome apparent after considering the following detailed specificationwhich covers preferred embodiments thereof in conjunction with theaccompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of arivet squeezer portion of a tool constructed according to the presentinvention in position to start a Rivbolt fastening cycle;

FIG. 2 is a partial cross-sectional view of the present tool withrelative positioning of the components therein being those at the startof the Rivbolt fastening cycle;

FIG. 3 is a partial cross-sectional view similar to FIG. 2 with apreload established by the tool;

FIG. 4 is a partial cross-sectional view similar to FIG. 2 with the toolcomponents in final rivet upsetting positions;

FIG. 5 is a diagrammatic representation of the present squeezer toolincluding its pumps, accumulator, and controls.

DESCRIPTION OF THE PRESENT EMBODIMENTS Referring to the drawings moreparticularly by reference numbers, number in FIG. 1 refers to the rivetsqueezer tool portion of a rivet squeezer tool 11 constructed accordingto the present invention. The tool 10 includes a generally C-shaped bodyportion 12 with a preload cylinder 14 connected to one end thereof and asqueeze cylinder 16 connected to the other. As can be seen, the tool 10can be hand held in engagement with the workpiece 18 while rivets suchas Rivbolts 20 are being installed to hold portions of the workpiece 18together.

In FIG. 2 it can be seen how a Rivbolt 20 is positioned in a drilledhole 22 in the workpiece 23 with a collar 24 in place around the tail 26of the Rivbolt 20. As the tool 10 is placed about the Rivbolt 20, apreload sleeve 28 including a collar mating annular groove 30 is placedin contact over the collar 24. The preload sleeve 28 is formed as eitheran integral or connected extension of a preload piston 32 which rides onthe inner cylindrical surface 34 of the preload cylinder 14 and on theouter cylindrical surface 36 of a post anvil 38 whose function will bedescribed hereinafter. The post anvil 38 extends from a plug member 40which seals one end of the preload cylinder 14. The post anvil 38 may beintegral with the plug member 40 or connected thereto as shown. Having aremovable preload sleeve 28 and a removable post anvil 38 isadvantageous as it allows widely varied types and sizes of Rivbolts 20to be installed with the same basic tool 10.

Sealing means such as O-rings 42 and 44 are included between the preloadpiston 32 and the surfaces 34 and 36 respectively to form a sealedchamber 46. The chamber 46 is supplied with hydraulic fluid at arelatively constant and predetermined pressure through a port 48 bymeans described hereinafter. Therefore, the piston 32 is normally urgedaway from the plug 40 with a constant predetermined force no matter howlarge or small the chamber 46 becomes.

Once the Rivbolt 20 and its collar 24 are properly positioned within thepreload sleeve 28, a head set 50 extending from a squeeze piston 52 andguided by a retainer ring 53 forming the inner end of the cylinder 16,is extended until it contacts the head 54 of the Rivbolt 20. The headset 50 may be replaceable as shown and it usually includes a shapedportion such as the depression 55 which is contoured to properly abutthe head The squeeze piston rides on the inner cylindrical surface 56 ofthe squeeze cylinder 16 and includes high pressure sealing means such asa T-seal 58, available from Parker Seal Company of Los Angeles, Calif,therebetween. The cylinder 16 and the piston 52 define a sealed chamber60 into which hydraulic fluid at high pressure is controllably admittedthrough a port 62. The squeeze piston 52 is normally biased away fromthe Rivbolt 20 by a spring 64 acting between the piston 52 and theretainer ring 53 so that the chamber 60 is usually of minimal size. Whenhigh pressure hydraulic fluid is admitted through the port 62, thepiston 52 and its head set 50 are driven toward the Rivbolt 20 until thehead set 50 engages therewith. From that point on as shown in FIG. 3,the extension of the head set 50 causes the preload sleeve 28 of thepreload piston 32 to be forced back against the aforementioned constantpressure in the chamber 46 with the preload force being applied from thehead 54, across the workpiece 23 to the preload sleeve 28. The extensionof the head set 50 continues until the preload piston 32 is driven backfar enough to allow the end 68 of the post anvil 38 to come in contactwith the tail 26 of the Rivbolt 20. An abutment surface 69 is formed onthe retainer ring 53 facing the piston 52 to act as a positive stoptherefor in case the tool 10 is inadvertantly activated without aRivbolt positioned in the working gap.

When the tail 26 of the Rivbolt 20 comes into contact with the anvil end68, a very rapid force build-up occurs across the Rivbolt 20. Thepressure in the chamber 60 is predetermined to apply sufficient forceacross the Rivbolt 20 to exceed the elastic limit thereof so the Rivbolt20 swells within the hole 22 to expand the walls of the hole 22 therebycreating residual compressive stresses which increase the fatigue lifeof the workpiece 23. As the force continues to build-up, the tail 26 ofthe Rivbolt 20 gradually mushrooms as shown in FIG. 4, increasing indiameter until it achieves a shape commensurate with a terminal upsetforce specified for the Rivbolt 20 and controlled by the pressuresupplied to the chamber 60. After the Rivbolt upset has beenaccomplished, the pressure in chamber 60 is relieved and the spring 64moves the squeeze piston 52 backward in the squeeze cylinder 16 toretract the head set 50 thereby allowing extension of the preload sleeve28 to the position shown in FIG. 2. It should be noted that thecross-sections of the cylinders 14 and 116 are thicker at the sidesconnecting to the C-shaped body portion 12 than at the opposite sides.This is done to optimize the strength of the tool while enabling it tobe used in tight places such as in the cramped workpiece 23 of FIGS. 2,3 and 4.

In some instances when the Rivbolt 20 is slightly too long, themushroomed tail 26 will expand outwardly far enough to seize on theinner surface 71 of the preload sleeve 28. Optional stripping means canbe provided to overcome this seizing by including a guide ring 72 whichthreadably engages withthe inner end of the preload cylinder 14 andwhich includes sealing means such as an O-ring 74 to form a seal againstthe outer surface 76 of the preload sleeve 28. This forms a sealedchamber 78 between the preload piston 32 and the guide ring 72. A port80 can then be used to conduct pressurized medium into the chamber 78 toforce the preload piston 32 back against the constant pressure in thechamber 46 until the piston 32 engages the inner surface 82 of the plug40.

If the mushroomed tail 26 of an overly long Rivbolt 20 is seized to thepreload sleeve 28, the action of pressurizing the chamber 78 causes thesleeve 28 to retract over the post anvil 38 which drives the mushroomedtail 26 out of the sleeve 28 and frees the tool 10 for its next cycle ofoperation. The stripping means can also be energized to increase the gapacross the tool 10 when positioning the tool in a cramped location.

FIG. 5 shows suitable means by which it is possible to provide thepreload pressure, the squeeze pressure and the stripping pressure to thetool 10. The constant pressure preload can be provided by any suitablemeans such as a gas-fluid accumulator 84 which is shown connected to thepreload chamber 46 through port 48 by a line 86. The accumulator 84, asshown, has a gas chamber and a fluid chamber which are separated by aflexible diaphragm 87. The preload pressure on the fluid in the chamber46 is set by charging the accumulator 84 to a predetermined pressurewith gas, which pressure transfers to the fluid therein, therebyproviding fluid at relatively constant pressure to the chamber 46.Typical design pressures applied to the chamber 46 are in the range of800 to 1,000 psi. The volume of the gas chamber should be relativelylarge with respect to the volume of the preload chamber 46 so thatmovements of the sleeve piston 32 which change the volume of the chamber46, do not appreciably change the volume of the gas chamber and hencethe preload pressure extablished thereby.

High pressure, in the design range of 10,000 psi supplied to chamber canbe provided by means such as an air-oil pump 88, which are connected toport 62 by a high pressure line 90. The air-oil pump 88 can be poweredwith relatively low pressure air in the range of 90 psi through line 92.In response to the actuation of a valve 94 thereon, the pump 88 pumpshigh pressure oil through the line 90 and the port 62 to the chamber 60.The'valve 94 as shown, includes a controlling pedal 95. Tilting of thepedal 95 in the direction shown by arrow 96 causes an increasing flow ofpressurized oil to be pumped to the chamber 60 while tilting in thedirection of arrow 98 causes dumping of the pressurized fluid. The pedal95 is usually spring-loaded in the neutral position shown to maintainany pressure in line 90 and chamber 60, thereby maintaining the squeezepiston 52 at whatever position it is in. A pressure gauge 100 toindicate the pressure in line 90 and the squeeze chamber 60 can beincluded although it is normally not required since air-oil pumps suchas pump 88 are easily controlled by conventional regulator or reliefmeans 101 to provide a predetermined maximum pressure.

When stripping means are employed in the present invention, a secondair-oil pump 102 similar to the pump 88 but restrained from such highpressure outputs is connected to the port 80 and hence the chamber 78 bymeans of an output line 104. The pump 102 is powered by low pressure airfed through line 106 from the source of low pressure air whose output isalso fed to the pump 88 by line 92. The valve 108 on the pump 102 issimilar to valve 94 but it is interconnected with the valve 94 to onlyapply stripping pressure to the chamber 78 when the pedal 95 of thevalve 94 is moved toward the extreme in the dump direction as shown byarrow 98. This causes automatic stripping of the sleeve 28 from theRivbolt 20 whenever the squeeze cylinder 60 is dumped to release thepressure therein. Although air powered pumps and accumulators are shownas the pressure generating means for the tool 11, any other device whichcan essentially duplicate their actions can be substituted as desired.

Thus there has been shown and described novel rivet squeezers whichfulfill all of the objects and advantages sought therefor. Many changes,alterations, modifications and other uses and applications of thesubject rivet squeezers will become apparent to those skilled in the artafter considering this specification and the accompanying drawings. Allsuch changes, modifications, variations and other uses and applicationswhich do not depart from the spirit and scope of the invention aredeemed to be covered by the invention which is limited only by theclaims which follow.

What is claimed is:

l. A tool for preloading and upsetting a rivet in a structure, the rivethaving a rivet head on one end, a tail on the other end and a collarwhich fits over the tail once the rivet has been inserted through thestructure, said tool including:

a body portion having first and second ends;

an upset anvil connected to said first body portion end adapted to abutone end of the rivet;

preload means for establishing a relatively constant force against thestructure connected to said first body portion end, said preload meansincluding a first chamber, a piston forming one end of said firstchamber, means to maintain a relatively constant pressure within saidfirst chamber to act on said piston, and a preload sleeve connected tosaid piston and formed around said upset anvil, said preload sleevebeing adapted to transmit force to the structure and including a ringshaped abutment surface facing said rivet drive member to abut thestructure and an annular groove adjacent to the inner edge of said ringshaped abutment surface for engaging the collar;

a rivet drive member slidably connected to said second end of said bodyportion facing said upset anvil; and

means to controllably advance said rivet drive member toward said upsetanvil and retract said rivet drive member from said upset anvil wherebysaid rivet drive member contacts one end of the rivet and drives therivet and the structure back against said preload means so that theother end of the rivet is driven back into abutment with the upset anviland is upset thereby.

2. The tool defined in claim 1 wherein said means to maintain arelatively constant pressure within said first chamber include:

a gas-fluid accumulator having a volume of pressurized gas substantiallygreater than the maximum volume of said first chamber; and

a line connecting pressurized fluid from said gas-fluid accumulator tosaid first chamber.

3. The tool defined in claim 1 wherein said preload sleeve includes:

an inner contoured surface adjacent to said annular groove, said innercontoured surface restraining the upset tail of the rivet to apredetermined shape.

4. The tool defined in claim 1 wherein said means to controllablyadvance and retract said rivet drive member include:

a hydraulic cylinder;

a drive piston in said hydraulic cylinder connected to said rivet drivemember;

means for controllably applying pressurized fluid to said cylinder toact on said drive piston to extend said rivet drive member; and

a spring operatively connected to retract said rivet drive member whensaid pressurized fluid is being released from said cylinder.

5. A tool for preloading and upsetting a rivet in a structure, said toolincluding:

a body portion having first and second ends;

preload means for establishing a relatively constant force against thestructure connected to said first body portion end;

an upset anvil connected to said first body portion end adapted to abutone end of the rivet;

a rivet drive member slidably connected to said second end of said bodyportion facing said upset anvil; and

means to controllably advance said rivet drive member toward said upsetanvil and retract said rivet drive member from said upset anvil wherebysaid rivet drive member contacts one end of the rivet and drives therivet and the structure back against said preload means so that theother end of the rivet is driven back into abutment with the upset anviland is upset thereby, said means to controllably advance and retractsaid rivet drive member including a hydraulic cylinder including acylinder housing connected on one side thereof to said body portion,said cylinder housing having a greater cross-sectional thickness at itsside adjacent said body portion than its cross-sectional thickness atthe side opposite therefrom, a drive piston in said hydraulic cylinderconnected to said rivet drive member, means for controllably applyingpressurized fluid to said cylinder to act on said drive piston to extendsaid rivet drive member, and a spring operatively connected to retractsaid rivet drive member when said pressurized fluid is being releasedfrom said cylinder.

6. The tool defined in claim 5 wherein said cylinder housing includes:abutment means for engaging said drive piston to restrict the distancesaid drive piston can move in the rivet drive member extendingdirection, said spring being a compression spring positioned betweensaid drive piston and said cylinder housing, whereby said abutment meansprevent the movement of said drive piston to a position in which saidcompression spring is compressed beyond a predetermined point.

7. The tool defined in claim 5 wherein said preload means include:

a preload chamber connected to said first body portion end;

a gas-fluid accumulator having a volume of pressurized gas substantiallygreater than the maximum volume of said preload chamber; and

a line connecting pressurized fluid from said gas-fluid accumulator tosaid preload chamber.

1. A tool for preloading and upsetting a rivet in a structure, the rivethaving a rivet head on one end, a tail on the other end and a collarwhich fits over the tail once the rivet has been inserted through thestructure, said tool including: a body portion having first and secondends; an upset anvil connected to said first body portion end adapted toabut one end of the rivet; preload means for establishing a relativelyconstant force against the structure connected to said first bodyportion end, said preload means including a first chamber, a pistonforming one end of said first chamber, means to maintain a relativelyconstant pressure within said first chamber to act on said piston, and apreload sleeve connected to said piston and formed around said upsetanvil, said preload sleeve being adapted to transmit force to thestructure and including a ring shaped abutment surface facing said rivetdrive member to abut the structure and an annular groove adjacent to theinner edge of said ring shaped abutment surface for engaging the collar;a rivet drive member slidably connected to said second end of said bodyportion facing said upset anvil; and means to controllably advance saidrivet drive member toward said upset anvil and retract said rivet drivemember from said upset anvil whereby said rivet drive member contactsone end of the rivet and drives the rivet and the structure back againstsaid preload means so that the other end of the rivet is driven backinto abutment with the upset anvil and is upset thereby.
 2. The tooldefined in claim 1 wherein said means to maintain a relatively constantpressure within said first chamber include: a gas-fluid accumulatorhaving a volume of pressurized gas substantially greater than themaximum volume of said first chamber; and a line connecting pressurizedfluid from said gas-fluid accumulator to said first chamber.
 3. The tooldefined in claim 1 wherein said preload sleeve includes: an innercontoured surface adjacent to said annular groove, said inner contouredsurface restraining the upset tail of the rivet to a predeterminedshape.
 4. The tool defined in claim 1 wherein said means to controllablyadvance and retract said rivet drive member include: a hydrauliccylinder; a drive piston in said hydraulic cylinder connected to saidrivet drive member; means for controllably applying pressurized fluid tosaid cylinder to act on said drive piston to extend said rivet drivemember; and a spring operatively connected to retract said rivet drivemember when said pressurized fluid is being released from said cylinder.5. A tool for preloading and upsetting a rivet in a structure, said toolincluding: a body portion having first and second ends; preload meansfor establishing a relatively constant force against the structureconNected to said first body portion end; an upset anvil connected tosaid first body portion end adapted to abut one end of the rivet; arivet drive member slidably connected to said second end of said bodyportion facing said upset anvil; and means to controllably advance saidrivet drive member toward said upset anvil and retract said rivet drivemember from said upset anvil whereby said rivet drive member contactsone end of the rivet and drives the rivet and the structure back againstsaid preload means so that the other end of the rivet is driven backinto abutment with the upset anvil and is upset thereby, said means tocontrollably advance and retract said rivet drive member including ahydraulic cylinder including a cylinder housing connected on one sidethereof to said body portion, said cylinder housing having a greatercross-sectional thickness at its side adjacent said body portion thanits cross-sectional thickness at the side opposite therefrom, a drivepiston in said hydraulic cylinder connected to said rivet drive member,means for controllably applying pressurized fluid to said cylinder toact on said drive piston to extend said rivet drive member, and a springoperatively connected to retract said rivet drive member when saidpressurized fluid is being released from said cylinder.
 6. The tooldefined in claim 5 wherein said cylinder housing includes: abutmentmeans for engaging said drive piston to restrict the distance said drivepiston can move in the rivet drive member extending direction, saidspring being a compression spring positioned between said drive pistonand said cylinder housing, whereby said abutment means prevent themovement of said drive piston to a position in which said compressionspring is compressed beyond a predetermined point.
 7. The tool definedin claim 5 wherein said preload means include: a preload chamberconnected to said first body portion end; a gas-fluid accumulator havinga volume of pressurized gas substantially greater than the maximumvolume of said preload chamber; and a line connecting pressurized fluidfrom said gas-fluid accumulator to said preload chamber.